Measuring of radioactive radiation emanating from an active ingredient applied to a carrier has attained increased importance, for example in medical laboratory technology.
A basic disadvantage of such measurements is found in that the radiation emanating from the carrier in general extends over a spatial angle of 2.pi., which corresponds to the surface of a hemisphere. Corresponding conditions prevail with respect to secondary radiation. The accuracy of the locality-sensitive measurement is inevitably harmed by this effect.
In known devices an attempt is therefore made to bring the entrance window of the locality-sensitive counting tube as close as possible to the carrier surface to which the radioactive substance has been applied to keep this effect as small as possible. However, this method inevitably has its limits, for example because of contamination of the underside of the detector or the danger of damage to the detector interior by the radioactive sample.
A known alternative to this is the use of a plate-shaped collimator with a plurality of through-bores or -slits extending perpendicular to its surface; such a collimator may be disposed between the carrier and the detector. The detector in such a device may be, for example, a two-dimensional proportional counting tube, as disclosed, for example, in German Published, Non-examined Patent Application DE-OS 37 35 296. Depending on the bore diameter or the width of the slits and the thickness of the collimator, only a very small fraction of the isotropic radiation is selected from the "available" spatial angle .OMEGA., which extends by the amount of .DELTA..OMEGA. at right angles to the carrier surface. By means of this, the particles/rays which extend "too obliquely" are eliminated and local resolution is increased. However, unfortunately, connected with the elimination of the particles/rays not desired for local resolution is the disadvantage that these "undesired" particles/rays cannot make a contribution to the counting rate of the detector and that because of this the percentage measuring sensitivity of the detector is reduced to the same extent as the ratio of the spatial angle area .DELTA..OMEGA. "selected" by the collimator to the total spatial angle .OMEGA.. This can be briefly expressed as: EQU I/I.sub.O =2.pi. (1-cos .THETA.),
where .THETA. is the angle of divergence of the radiation. Typical values for the ratio I/I.sub.O are approximately 1%, which indicates a corresponding unsatisfactory reduction of the detector sensitivity.